Until recently, cellular telephone (cellphone) car kits (i.e., devices to mount a cellphone handset into a “nest” in a vehicle for hands-free user operation of the cellphone) were low volume products whose main users were “road warriors” and other business customers willing to pay several hundred dollars for such equipment. Recently however, some European countries and American states and localities have introduced legislation banning the use of cellphone handsets by drivers. As a result, the market for car kits has started to expand rapidly, and this trend is expected to continue. As car kits have moved into mass markets, the demand for a much lower cost solution is driving cellphone handset manufacturers to seek lower cost architectures. Simultaneously, the Universal Serial Bus (USB) protocol is becoming the standard interface on low-end cellphone handsets. The USB and/or USB On-The-Go protocol (the Universal Serial Bus 2.0 On-The-Go (USB OTG) Supplement, Revision 1.0, published December 2001 and hereby incorporated by reference in its entirety) is anticipated to be the primary data interface protocol for almost all cellphone handsets within a few years.
A typical cellphone handset with USB (or USB OTG) interconnecting capability has two connectors for interconnection to external devices. The two connectors are (i) a “jack plug” socket for attachment of an earpiece or other audio interface device to the cellphone and (ii) “sprung contact” type connector pads (or pins). The sprung contact connector is implemented for the USB interface (or port), battery charging to the cellphone handset battery, analog output to a cellphone car kit, etc. A typical cellphone handset that implements the USB OTG protocol includes a digital circuit (i.e., a baseband processor ASIC) that is configured to receive and present the digital portion of the cellphone signals and a physical layer interface (PHY) in an external IC that is configured to receive and present the analog portion of the cellphone signals.
The cost of a cellphone car kit is often more than the cost of the phone itself. The car kit incorporates a USB host that receives USB digital audio output packets, and converts the digital audio into analog form and plays the audio on the car kit loudspeaker(s). The car kit USB host also receives analog audio signals from the car kit microphone(s), converts the audio analog signals into digital form and transmits them over the USB bus to the cellphone handset.
A number of conventional solutions to reduce the cost and simplify the cellphone handset to car kit interconnection have been proposed. One proposed solution is to add two (i.e., mono in, mono out) or three (i.e., mono in, stereo out) pads/pins to the “sprung contact” connectors typically used by handset manufacturers to interface between the handset and the car kit. However, the implementation of additional connector pads/pins is unattractive because of the increased cost to the cellphone handset and the increased potential for unreliable contacts.
Another proposal is to add a jack plug connection to the car kit for interfacing to the audio socket on the handset. However, adding a jack to the car kit is unlikely to receive a favorable response from consumers who would like to be able to connect the handset to the car kit simply by placing the handset in the “nest” provided in the car kit. The added jack also has the potential for unreliable contacts.
Another proposal is to share the D+, D− and either the ID or the VBus pins found in the USB or USB OTG interface by capacitive coupling the audio signals within the cell phone to the USB or USB OTG signal pins. However, the disadvantages of capacitive coupling the audio signals within the cell phone to the USB or USB OTG signal pins include (i) the audio signals to be coupled to the USB pins must be switched elsewhere within the cellphone handset and (ii) having additional capacitance connected to the USB signaling pins is undesirable, even when the unconnected side of the capacitance is “floating”. Additionally, the conventional proposals where external multiplexing is implemented do not provide a solution that is compliant with High Speed (HS) USB (i.e., 480 Mb/s) since the USB connection requires very tightly controlled characteristics on the D+ and D− signal conductors.